The present invention relates to mobile communication systems, and more particularly to initial cell search techniques in mobile communication systems
Mobile communication systems, such as cellular communication systems, allow mobile user equipment (UE) to communicate wirelessly by establishing a wireless (e.g., radio) link between the UE and one of a number of base stations (BS) which are geographically distributed throughout a service area. Mobility is provided by means of protocols that enable the UE to be handed off from a first BS to another as it moves from the coverage area of the first BS to the coverage area of the other BS.
The various base stations are connected (e.g., by means of wireless and/or wired links) to a public land mobile network (PLMN), which provides the necessary infrastructure for servicing calls. The PLMN also typically has connections to public switched telephone networks (PSTNs) to enable calls to be routed to wireline communication devices not associated with the PLMN.
Even when it is not actively engaged in a call, UE that has been switched on for a while typically “camps on” a control channel of a suitable base station. This enables the UE to be informed and to respond when it is the recipient of a call, and also enables the user to quickly initiate his or her own calls.
However, when the UE is first switched on, or when the network has been lost for a long time (e.g., when the UE has been out of a coverage area for a long time), the terminal must perform an initial cell search procedure to identify which cells (each associated with a base station) are available. The UE will select the best of the available cells that it finds from the search.
Because the UE might “wake up” essentially anywhere (e.g., in a country different from the one in which it was last switched on) the initial cell search typically involves searching for the presence of control channels throughout an entire available radiofrequency band. One hindrance in this respect is the fact that the accuracy of the UE's oscillator can vary, primarily due to temperature fluctuations of the frequency generating components. As long as the internal temperature is stable and nothing else happens that affects the frequency, the accuracy (and therefore also inaccuracy) of the generated frequency will be stable. Changes of the UE's internal temperature can be due to a change of activities in which the UE is engaged (e.g., the UE starting to receive or transmit data) and/or due to changes in the environment surrounding the UE.
Because of the possibility of varying frequency inaccuracy, the conventional initial cell search procedure must monitor not only the center frequencies of potential control channels within the available radiofrequency band, but must also monitor some number of frequencies on either side of the “desired” center frequencies, in case frequency inaccuracy causes there to be a wide disparity between the UE's generated frequency and the accurate frequency being used by a transmitting base station.
An example relating to the Wideband Code Division Multiple Access (WCDMA) standard of mobile communication will now be presented to illustrate a conventional initial cell search process. The invention to be presented herein should not be considered limited to use only in WCDMA systems, however, since it is equally applicable to other mobile communication systems as well.
A conventional initial cell search technique typically assumes that the frequency inaccuracy is large, on the order of about 10 parts per million (ppm) which means ±20 kHz on the 2 GHz band, when searching for carriers. A good level of accuracy in frequency generation makes coherent integration of the received signal possible, and thereby good performance. But, when frequency inaccuracy is large, the coherence in the receiver is deteriorated, and thereby so is the receiver's performance. This leads to a long search time being required.
To improve the search time, one approach involves using several searches with different center frequencies, where each of the searches assumes a better level of accuracy. For example, it is possible to compensate for a ±20 kHz inaccuracy by performing four searches, each assuming ±5 kHz inaccuracy. The searches are performed on the carriers fc=±5 kHz and fc=±15 kHz. This approach has a drawback in that it takes about four times as long as a single search with a frequency inaccuracy that is less than 5 kHz.
FIGS. 1a through 1c are flow charts that illustrate a conventional initial cell search algorithm that utilizes the just-described approach of searching multiple center frequencies on either side of the actual desired center frequency. The initial cell search procedure may be applied, for example, in the Universal Mobile Telecommunications System (UMTS). FIG. 1a illustrates an overview of the entire procedure. The goal of the search is to identify a carrier frequency that is being used by a cell associated with a target PLMN. To start out this search, an initial search list is put together that includes all valid UMTS Absolute Radio Frequency Channel Numbers (UARFCNs) (block 101).
One aspect of the approach is that discovering a cell on one center frequency may make it possible to eliminate other neighboring frequencies from a subsequent search, which has the effect of speeding up the overall search time. Accordingly, to increase the likelihood of finding a cell, the initial search procedure first performs a history list search (block 103). The history list may consist, for example, of some number (e.g., five, although this number is not essential) of most recent frequencies on which a suitable cell was found.
FIG. 1b is a flowchart of an exemplary history list search algorithm 103. In this example, the history list consists of some number of the most recent frequencies on which a suitable cell was found. As shown in block 121, the list is continuously updated each time a new PLMN/frequency is found. Upon deactivation/powering off of the UE, the history list is stored in a non-volatile memory for later use when the UE is again powered on.
To begin the actual searching, the first UARFCN in the history list is selected (block 123). Then a search loop is entered that runs a cell search on the selected UARFCN and removes the UARFCN from the initial search list (block 125). If a new cell is found, (“YES” path out of decision block 127), information received from the cell is used to determine whether it is from the target PLMN (decision block 129). If the cell is from the target PLMN (“YES” path out of decision block 129), then the search algorithm need not look further.
If, however, the found cell is not from the target PLMN (“NO” path out of decision block 129), all UARFCNs that are ±3 MHz from the UARFCN associated with the found cell are removed from the initial search list (block 130). Since removing these UARFCNs from the initial search list will prevent these carriers from being searched in later passes of the initial cell search, this has the effect of speeding up the overall search time.
Following block 130, or if a cell had not been found on the selected UARFCN (“NO” path out of decision block 127), a determination is made whether the last UARFCN in the history list had been selected (decision block 131). If not, (“NO” path out of decision block 131), the next UARFCN in the history list is selected (block 133), and the loop is repeated by returning processing to block 125. Determining that the last UARFCN in the history list had been selected (“YES” path out of decision block 131) constitutes the end of the history list search 103.
Returning to FIG. 1a, upon completion of the history list search, the next activity involves processing the downlink (DL) frequency band as follows.
First, the initial search list is reduced by filtering out frequencies based on their Received Signal Strength Indicators (RSSIs) (block 105). This filtering involves:                making an RSSI scan on each UARFCN in the initial search list;        for any of the frequencies that are ±100 kHz, ±300 kHz, ±500 kHz from the center frequencies in the DL frequency band, removing all UARFCNs from the initial search list that satisfy RSSI≦−100 dBm;        for any of the frequencies that are not        ±100 kHz, ±300 kHz, ±500 kHz from the center frequencies in the DL frequency band, removing all UARFCNs from the initial search list that satisfy RSSI≦−95 dBm.By removing frequencies that are not likely to result in a found cell, the searching effort is further reduced to testing only the most probable carriers in the band.        
Finally, the resulting (filtered) list is searched 107. FIG. 1c is a flowchart illustrating an exemplary searcher 107. The approach taken is to search the most probable frequencies first, and then to search all other frequencies in the search list. Referring now to FIG. 1c, the center frequency to be used, fc, is set equal to a carrier frequency in the DL frequency band (e.g., fc=2112.5 MHz) and the UARFCN whose frequency is fc−100 kHz is selected (block 141).
If the selected UARFCN is in the initial search list (“YES” path out of decision block 143), then a cell search is run on the selected UARFCN and the selected UARFCN is removed from the initial search list (block 145) in order to prevent if from being searched a second time. If the cell search found a new cell (“YES” path out of decision block 147), then information received from the cell is used to determine whether it is from the target PLMN (decision block 148). If it is (“YES” path out of decision block 148), then no further searching need be performed.
However, if the found cell is not from the target PLMN (“NO” path out of decision block 148), then all UARFCNs that are ±3 MHz from the selected UARFCN associated with the found cell are removed from the initial search list (block 149).
Following this, or if no new cell was found (“NO” path out of decision block 147) or if the selected UARFCN was not found to have been in the initial search list (“NO” path out of decision block 143), then an algorithm is performed that either selects a next UARFCN to be used in a subsequent pass of the loop, or else the initial search is terminated (block 151). To perform a next pass of the loop, processing returns to decision block 143.
The processing associated with block 151 (i.e., either selecting a next UARFCN to be used in a subsequent pass of the loop, or else terminating the initial search) can be performed in any of a number of ways. For example, carriers can be sorted in RSSI order (with strongest carriers appearing first) and searched in that sort order until all carriers have been selected for searching, at which point the initial search is terminated).
In one embodiment, the entire frequency band is divided up into a number of smaller frequency bands. For each of these smaller frequency bands, a known center frequency is selected, and then block 151 ensures that each of the carriers defined by fc±100 kHz, fc±300 kHz, and fc±500 kHz is at some point selected for searching.
For more information about known initial cell search techniques, the interested reader is referred to US Pub. No. US 2004/0203839 A1, published on Oct. 14, 2004 (Ostberg et al., “Mobile Terminals and Methods for Performing Fast Initial Frequency Scans and Cell Searches”).
One problem with the conventional initial cell search algorithm is that the search on all carriers takes a long time. In some cases it may take several minutes before it finds an allowable PLMN. One consequence this has on the UE is that time to registration to the network is long, which in turn means that the time from when the UE is first powered on until a call can be made is long. This negatively affects the user of the UE.
Another affect on the UE is that electric current consumption when the initial cell search algorithm is performed is high.
It is therefore desirable to provide initial cell search apparatuses and methods that are capable of more quickly identifying a cell associated with an allowable PLMN.